You can’t fully appreciate your vision without breaking it down into parts.
When everything is working correctly, you don’t really notice it.
You may not even realize how many systems are running in parallel to help you perceive
the world.
So let’s throw it all out the window and build it back up from nothing.
We’ll start this off by talking about the obvious form of blindness.
Total and complete blindness.
This is usually caused by having damaged eyes or optic nerves, either physically from an
accident, developmentally, or from a disease.
So what do you see when you’re blind?
Nothing.
Not blackness, not darkness, just nothing.
Tommy Edison has been vlogging and answering questions about what it’s like to be blind
for years.
They always want to know, what do you see?
Like, you must see something, you have to see something.
No, I don’t see anything, and then it’s a five hour conversation.
And in the comments, constantly, people try to tell him that he sees black.
As if they’d somehow know better than he does.
Sighted people always talk about, well then you must see black.
Well no, because you have to see to know what black is, right?
So therefore, I can’t see black, it’s just nothing.
He’s been blind since birth so he has no reference point for white, black, or any other
color, or even what lightness and darkness is.
He sees nothing.
And I understand why that would be confusing for a sighted person, because when you close
your eyes, you see darkness or blackness, or sometimes residual opposing col… it doesn’t
matter I’m getting ahead of myself, you see stuff, you know what I mean.
But as we’ve explored previously, when you close one eye, you don’t see half black.
Your brain just shuts off all information coming from that eye – You see this.
For a quick demonstration, close one eye, cover that eye, then open your eye when you start
to read the second line of text on screen.
You should see a big difference, and if you don’t…
try again or something.
The point is, that what your brain is doing to that one eye when it’s not in use is
what Tommy’s brain does to both eyes.
He sees nothing, because his brain is receiving and processing no information.
You can kind of experience this when you’re day dreaming and you’re not really paying
attention to what you’re seeing, like when you’re drifting off to sleep or driving.
If I went up to a blind person and asked: How many fingers am I holding up?
I’m blind –
Yes I know but… could you just take a guess?
By pure chance alone, they will get it right 20% of the time.
What is wrong with you guys.
But if I went up to a blind person and asked: What color shirt am I wearing?
I’m still blind – Yes I- we’ve been through that already,
could you just take a guess please?
They will understandably almost never get it right since there’s nearly an infinite
amount of colors.
But there is a form of blindness where they will get it right almost always.
It’s called cortical blindness, and it’s when the eyes and optic nerves are completely
intact and functional, but the visual cortex, also known as Area V1, is damaged, either
from an accident or injury, stroke, or a neurodegenerative disease.
The point is it’s always acquired later in life, you’re not born with it.
People with cortical blindness cannot consciously see – if you ask them, they will tell you
that they are blind, and for all practical purposes, they are.
But if I were to do something like…
Just like you just did, they will flinch, which is strange right?
Although it’s a little stranger for you because you’re watching a video but let’s
not split hairs.
How can someone who is blind still see and react to stimulus, but not… see?
Well oddly enough, it’s call Blindsight, and it’s caused by the fact that your occipital
lobe is not the only part of the brain that processes vision – in fact it’s also not
even the first part of the brain.
The optic nerve first travels through the subcortical structures, more specifically
the thalamus, and even more specifically the Lateral Geniculate Nucleus, or LGN.
And this processes basic vision before the information ever gets to your cortex and conscious
awareness.
This is why you flinch a split second before you even realize something is flying at your
face.
If you ask someone with blindsight why they flinched, they won’t be able to give you
an answer.
Their brain will make something up to explain their actions like
I just felt like something was coming.
But it’s because they can see - at least very basic things.
They can correctly guess colors, orientation of objects, direction of movement, and even
facial expressions like being happy or angry.
Almost always, like well over chance.
If you’re not impressed yet, here’s a guy who is cortically blind.
He will tell you he is blind and he definitely is blind.
But here he is walking through a hallway that he’s never been down before, avoiding obstacles
and objects without touching them.
I know what you’re thinking – this is some real Daredevil shi-.
But it’s not, because Ben Affleck’s eyes were damaged, not his visual cortex.
So let’s turn the visual cortex back on and talk about an interesting disorder known
as Balint’s Syndrome.
There are three parts to Balint’s.
First, is apraxia.
This is the inability to voluntarily focus or fixate your eyes.
Now when I first say that you might think it looks something like this… ugh.
But no, it actually looks something like this.
Look at the keys please.
I am.
Would y… would you like to try again?
They can’t point their eyes at anything that they want to.
They can shift their attention, which is different, but you already know that.
You’re able to focus on and watch one thing but pay attention to another.
Well that’s what they’re always doing… but they don’t know it, they think that
they’re looking at it.
The second is ataxia, which is the inability to guide your hand to an object using visual
information.
Reach out and grab the keys please.
Would… would you like to try again?
It’s not a depth perception issue, because they’ll be too far forward or back, left
or right, they'll be all over the place.
And I’m not just picking fun, while patients can see the object, they can name the
object, they just can’t interact with or properly attend to it.
And that’s the thing, it’s an attention issue, not a visual issue.
They’re receiving all of the information, but they’re not processing it correctly.
Which brings us to the third part, simultagnosia.
You’ve heard the saying “Can't see the forest through the trees."
Well someone with Balint’s and simultagnosia can only see one tree at a time and they are
incapable of seeing the entire forest.
So basically, they can only attend to one object at a time…
What does that mean?
It means that if I’m holding up my keys, they can either see me, or my keys… but
not both… and certainly not everything else.
Again, they’re receiving all of the information, but they can only attend to one thing at a
time.
The opposite of this is visual agnosia, when you can see an object but you can’t recognize
or identify it.
The form of this that you’re probably most familiar with is Prosopagnosia or Face Blindness,
which I’ve discussed before.
But more generally, there are two types, apperceptive, which is when I hold up an object and ask
you to identify it, but-
This is not aphasia where you forget the name of the object, you simply perceive it as a
colorful shape. It has no meaning as an object itself.
-but you just can’t.
You can describe it.
It’s a black bar with some teeth…
But you can’t name it.
And associative, which is when you can name it, but you have no idea what the meaning
or purpose of the object is.
It’s a comb.
Right, what’s it used for?
Uh... It’s when…
The interesting thing is, this only affects vision – and if the patient holds the object,
the immediately know what it’s for.
Oh, yeah!
But taking this to the extreme, we have visual neglect.
This is when sensation and perception work just fine, but the ability to attend to any
of it just isn’t there.
This usually only occurs in ha- whoa wait, okay go back, go back.
When you were a kid, you were told that your left eye connects to your right hemisphere
and your right eye connects to your left hemisphere.
Yeah well, you were also told that Santa and the Tooth Fairy were real, good things happen
to good people and bad things happen to bad people, and if you keep making that face it’ll
stick that way.
The truth is… much more complicated.
It’s not your eyes that cross over, it’s your visual field.
So the entirety of your left visual field connects to your right hemisphere, and vice
versa, and yes it’s also upside down.
It’s upside down for the same reason everything is upside down when you look into a spoon,
everything gets inverted… but let’s not overcomplicate this.
So anyway, how do visual fields work?
The left side of your left eye connects to your right hemisphere, this is called contralateral,
and it’s the part you all know.
But the left side of your right eye also connects to your right hemisphere, and this is called
ipsilateral.
And the right side of your left eye and right side of your right eye connect to the left
hemisphere.
Did I just blow your mind or give you a headache?
I know it’s complicated, but here’s an easy way to visualize it.
Here are the two visual fields or your eyes.
If you lose your left eye, you still maintain three-quarters of your visual field, because
of how much your eyes overlap.
Both your hemispheres still process the information.
But in most cases of visual neglect, you lose one hemisphere, which means you lose half
of your entire visual field, half from each eye.
So you can still physically see it, but you can’t interact
with or attend to any of it.
So if I were to hold up two objects, you’d only be able to name one of them… and if
I were to ask you to hold up your hand and divide your visual field in half, it would
be off center.
Enough of the scary stuff, let’s assume everything is on, everything is working, you
can attend to all of it… but for funsies, let’s take away something else.
Let’s take away Area V5, also known as Area MT.
Now, when I do something like that… things just pop into existence…
Congratulations, you have Akinetopsia.
This is the inability to perceive motion.
Remember the t-rex from Jurassic Park?
Ahhh!
Don't Move! It can't see us if we don't move!
Yeah, that’s not true and not possible… but the opposite definitely is.
People with damage to Area MT can’t keep track of large movements.
If you’re a fan of a fairly popular drug that is gaining increasing legality you’ve experienced
this effect, where things just seem to happen in frames rather than in one fluid motion.
This is partially because it’s a depressant so it lowers your perceived frames per…
you know what, just don’t do drugs… mmk?
Okay.
So alright, let’s get to something everyone can understand… in your eye, you have cones
and rods.
Oh snap it’s the color episode!
Finally!
Anyway to repeat a point I’ve made before, if cones are the pixels of your eye, rods
are the backlight.
You can’t only see with rods, because it would look something like this.
Which is terrifying.
It’s just the presence or absence of light, there’s virtually no definition.
Luckily, we have cones, so we don’t live in that nightmarish fever dream.
Most of us have three types, giving us full color vision, but what if you only had one?
This is called Achromatopsia, which just means the lack of color vision.
If you only have one cone, that doesn’t mean you only see in that one color, it’s
not monochromatic, it’s achromatic.
The reason night vision goggles are monochromatic is because it’s a screen that’s on purpose
only displaying in green.
You need at least two cones to differentiate between different wavelengths of light and perceive color.
if you only have one, it’s just yes there is light or no there isn’t.
And, while you could say it’s all shades of gray – I really hate that that book ruined
that phrase – it’s not one color, it’s no color.
So dogs, don’t see black and white, because they have two cones, and when you have at
least two, you can see color.
Ferrets on the otherhand are true color blind, because they only have one cone.
Ugh where am I?
Why did he wake me up this time?
Oh no, he’s talking to that stupid mechanical eyeball again.
He knows it’s not alive right?
Does he think it’s alive?
I guess he needs to practice talking to something if he ever wants to talk to a girl again…
I haven’t seen him talk to a girl in years…
… Wheatley….
Outside…
Wait what?
We’ll go outside in a second.
Anyway, from his perspective, he doesn’t see in color, since he only has one cone-
No I don’t think you understand, I don’t want to go outside, we go through this every
single time…
... Outside…
Bath time…
What?!
I just had a bath, I don’t want to go outside!
Okay we'll go outside geez...
Alright I guess he didn’t really want to go outside…
Achromatopsia can also be caused by damage to Area V4, which is the color center.
And just like with neglect and cortical blindness, it can sometimes only occur in one hemisphere
giving you this weird half-color vision.
When people lose their ability to see color completely, the often fall into depression
and end up committing suicide because of how bleak the world literally looks.
Well there’s a happy thought.
So let’s talk about dogs, people, and colorblindness!
Alright, alright, easy with the effects....
What just happened?
Okay nope, if this is what happens when I talk about color, forget it!
So what form of blindness do you find most terrifying?
Let me know down in the comments.
Will the color episode be next?
I don't know, so be sure to attend to that subscribe button.
In the meantime follow me on Facebook and Twitter, and join the conversation on the
subreddit...
What's the matter Wheatley, c'mon good boy...
Knob-end.